The more you search in (online) Korean archives, the more you will find of course. Here is a discovery from the (Korean) Law Archive regarding education revenue stamps from the Provincial Office of Education (POE) of Gangwon-do (강원도교육청). These stamps were in use from the mid-1990s until 1 January 2013, when they were phased out.
1994-2013
The file is the first attachment to the revenue stamp ordinance issued by Gangwon-do POE and shows a series of stamps with the following values: 100 won, 500 won, 1000 won, 5000 won and 10000 won. This file is shown with the 2010, 2011 and 2013 versions of the ordinance. The stamps were probably issued earlier than 2010: the first version of this ordinance is from 1994.

Value | Colour (Korean) | Colour (English) |
---|---|---|
100 won/원 | 연두색 | green |
500 won/원 | 갈색 | brown |
1.000 won/원 | 쑥색 | olive green |
5.000 won/원 | 주황색 | orange |
10.000 won/원 | 붉은보라색 | red purple |
For the complete ordinance see for instance the 2008 version of the text on the National Law Information Center website. The attachments including the one with the stamps is at the bottom of the page, look for “[별지 1] 강원도학예수입증지 도형”.
The ordinance also contains other interesting attachments. Take for instance the cancellation:
Or else these tally sheets which also show the values of the stamps:
1965
That is not all however. Another archive, the NAK, has a file showing a listing of revenue stamps from 1965. It’s not easy to read, but the values listed are 1 won, 2 won, 5 won, 10 won, 50 won and 100 won.

It’s too bad these older scans don’t show any of the stamps, it is all text. But at least it is proof these stamps existed at the time. The file is the third page in this document set.
There will be more of course, either in the NAK or NLIC archives or even elsewhere, from the